Richard Menis

Demonstration at sea of the decomposition-of-the-time-reversal-operator techniquea)

This paper presents a derivation of the time reversal operator decomposition (DORT) using the sonar equation. DORT is inherently a frequency-domain technique, but the derivation is shown in the time-frequency domain to preserve range resolution. The magnitude of the singular values is related to sonar equation parameters. The time spreading of the time-domain back-propagation image is also related to the sonar equation. Noise-free, noise-only, and signal-plus-noise data are considered theoretically. Contamination of the echo singular component by noise is shown quantitatively to be very small at a signal-to-noise ratio of 0dB. Results are shown from the TREX-04 experiment during April 22 to May 4, 2004 in 94m deep, shallow water southwest of the Hudson Canyon. Rapid transmission of short, 500Hz wide linear frequency modulated beams with center frequencies of 750, 1250, 1750, 2250, 2750, and 3250Hz are used. Degradation caused by a lack of time invariance is found to be small at 750Hz and nearly complete a...

The effect of coherence and noise on the decomposition of the time reversal operator

Active sonar in shallow water in shallow water is often reverberation-limited and the detectability is often limited by the presence of too many false alarms. The decomposition of the time reversal operator (DORT) is a method that potentially alleviates this problem by separating echoes from different depths in the water column. For example, DORT can separate a target in the water column from reverberation on the bottom. DORT requires a set of echoes recorded on a line array that result from a set of independent transmissions from a source array. A short derivation of DORT using the sonar equation is given. Because DORT is inherently a frequency-domain method, the time-frequency domain is derived to implement the algorithm on the data. Lastly, the similarity of DORT to adaptive beam forming is shown. In this paper, data taken on the Atlantic shelf, east of Cape May, NJ, during Geoclutter 03 and TREX-04 experiments, is processed and shown. The data was taken with a 64 element vertical line array of source-receiver elements. The target was an echo repeater using an XF4 source from 500 to 2500 Hz or an ITC 200 source from 2500 to 3500 Hz. The data cover six 500 Hz-bands from 500 Hz to 3500 Hz. The data are processed using DORT in the time-frequency domain. The analysis produces singular values in the time-frequency domain and in the time-delay domain. It also produces singular vectors that are used with a broad-band propagation model to form back-propagation images in the range, depth, frequency or range, depth, time domain. The analysis shows that the limiting factors in this data set arise from 1) motion that causes a lack of time-invariance, 2) additive noise and 3) the independent transmission scheme. The lack of time invariance is shown to spread the echo energy into several singular indices. Additive noise is shown to contaminate the singular values and back-propagation images. The particular transmission scheme used, time division multiplexed LFMs, is shown to create large side lobes in the time domain. Alternative transmission sequences, as well as alternative source and receiver orientations, are discussed

Application of DORT to active sonar

Active sonar in shallow water is often reverberation-limited and the detectability is often limited by the presence of too many false alarms. The problem of improving detection, and classification, in shallow water is being worked on in several different ways. The time reversal operator decomposition (DORT) is a technique that has recently been applied to the problem of discriminating echoes in shallow water based on the different depths of the scatterers. DORT uses scattering data from a multiple source and multiple receiver sonar arrangement to separate scatterers that are resolvable by the source and receiver arrays. DORT is the application of the singular value decomposition of the frequency-domain data. This paper presents a derivation of DORT from the sonar equation. DORT is inherently a frequency-domain technique. In order to preserve range-resolution, the sonar equation is transformed into the time-frequency domain. With that representation, DORT can be applied to the frequency domain signal within a range resolution cell. Following the derivation of DORT from the sonar equation, numerical simulations are shown that demonstrate the depth resolution of a vertical line array of sources and receivers. Sufficient depth resolution is shown using few sources at frequencies near 500 Hz in water with a depth of 100 m. Problems that are encountered with the implementation of the technique are discussed. Target motion causes leakage of signal energy into several singular values. Motion of the source or receiver is shown to have little effect. The problems and constraints that arise from different multiplexing techniques, including frequency, code and time division, are shown. Results are shown with data taken on the Atlantic shelf, east of Cape May, NJ, during Geoclutter 03 and TREX-04 experiments

Detection in shallow water using broadband—DORT

The decomposition of the time‐reversal operator (DORT) [Prada et al., J. Acoust. Soc. Am. 99, 2067–2076 (1996)] has been extended into a coherent, broadband method. Broadband DORT has also been shown to isolate resolvable scatterers at various depths and ranges in a bistatic, active sonar in shallow water. Results are shown from the application of DORT to sea data taken in an area south of Hudson Canyon off the New Jersey coast during Geoclutter II. The vertical source/receiver array with 56 hydrophones spanning the water column was operated between 3.0 and 3.5 kHz. The elements were divided into four groups, with each group acting as a coherent, broadside source. Two methods were used for exciting the separate channels. One method was the use of subsequent LFMs and the other was the use of simultaneous transmission of four pseudorandom‐noise signals. The target was a midwater column echo‐repeater. Results are compared with modeling based on in situ environmental measurements during the experiment. [The a...

Measurements and modeling of midfrequency clutter from fish aggregations over Georges Bank in the Gulf of Maine.

Broadband midfrequency acoustic measurements of fish backscattering were made with two complementary sonar systems in September 2008 at the northern slopes of Georges Bank in the Gulf of Maine. One system, towed near the surface, was downward‐looking (short‐range: < 0.2 km) and was used to image fish at high vertical resolution throughout the water column. The other system was horizontal‐looking (relatively long‐range: 1–10 km), consisting of a vertical source array and a horizontal receive array, and was used to image fish at high horizontal resolution. In this talk, the focus is on characterizing the longer‐range data. Spectrogram analysis showed that the echoes had resonance frequencies in the 2–4 kHz band, consistent with scattering by Atlantic herring, while examination of normalized match‐filter output revealed strong, spatiotemporally variable clutter. The associated probability‐density functions (PDFs) exhibited variable non‐Rayleigh behavior. Several PDF models were fit to the data, with the K an...

Measurements of signal spread and coherence on the New Jersey Shelf and in the Straits of Sicily using time‐forward and time‐reversed signals

Mid‐frequency shallow‐water propagation measurements were made at a variety sites on the New Jersey Shelf and in the Straits of Sicily during three joint trials with the SACLANTCEN (Boundary Characterization: 2000–2002) in order to extract measures of signal spread and coherence, and to evaluate the spatial robustness of time‐reversal techniques. The experiments had the NRV Alliance periodically transmitting a 1‐s, 200‐Hz‐bandwidth LFM while it traversed an arc about a stationary platform, which captured these signals and transmitted back time‐forward and a set of time‐reversed versions of them, as well as transmitting its own version of the original signal. Analysis of normalized matched‐filter data indicate that while the two‐way time spreads were generally modest, marked decorrelation was observed and that time reversal did well in reconstructing the original impulse response of the LFMs. As the various stored time‐reversed versions of the original signals corresponded to different source–receiver path...

Moment-based method to statistically categorize rock outcrops based on their topographical features

Geomorphological formations on the ocean seafloor are spatially complex objects. Rock outcrops in particular can be spatially compact, and highly anisotropic in their large- and small-scale topographic structure, and represent a significant source of clutter for short-range active sonars operating in shallow water. This paper describes a physical moment-based statistical method for categorizing rock outcrops based on their topographical characteristics, in particular isolating its facet-like features. The correspondence of these features to clutter-generating objects is then substantiated for a short-range, high-frequency shallow-water scenario, where the ensonified area is a fraction of outcrop size, via comparisons with high-fidelity acoustic backscattering predictions.

Time‐reversal operator target focusing using optimal beam sets

Time‐reversal operator (TRO) methods can be used to focus sound on an ocean target using a time‐reversal mirror. In order to increase the signal‐to‐noise ratio, the TRO can be measured by transmitting simultaneously the entire array instead of transmitting source by source. The so‐called beam‐space. TRO is measured by transmitting a set of orthogonal beams. Acoustic propagation will be simulated in a realistic range and time‐dependent ocean environment measured during the TREX04 experiment [Gaumond, J. Acoust. Soc. Am. (in press)]. This paper will compare four different sets of beams: Hadamard, phased planar, spatially phased, and modal. The first two methods require no a priori environmental knowledge, the third requires only waveguide depth, and the final requires a sound‐speed profile and geoacoustics measured at the TRM. Results will be discussed in terms of signal gain, complexity to implement, and coherence in an inhomogeneous ocean environment varying in both time and space. [Work supported by the ...

Echo detection enhancement using multiple guide sources in shallow water

The use of a guide source has been proposed as a way of compensating for multipath by forming a spatial‐temporal cross correlation of the received target and guide source signals across a vertical array in the frequency domain [Siderius et al., J. Acoust. Soc. Am. 102, 3439–3449]. This processing has the effect of creating a virtual receiver at the guide source position. In general, the performance of a virtual receiver degrades if the spatial integration is not carried out over the span of the array with significant signal. In our study, we have pursued an alternative approach of using guide sources which does not require this integration in general. The guide source signal is simply used as a matched filter. Although this does not correspond to a virtual receiver, it is useful as a means of improving active or passive detection of signals in noise. In general, the signal gain using this alternative technique is dependent on the guide source position. To compensate for this, we construct a separable‐kern...

Backpropagation image analysis of broadband decomposition of the time reversal operator (DORT) data from TREX‐04

Single‐frequency DORT is a method of isolating scatterers with a multiple‐source/multiple‐receiver system [C. Prada et al., J. Acoust. Soc. Am. 99, 2067–2076 (1996)]. Broadband DORT overcomes problems of frequency fading and dispersion by exploiting the response of singular values across the entire signal bandwidth. In spring 2004, the Time Reversal Experiment, TREX‐04, collected data for the evaluation of the broadband DORT technique. An 80 m vertical source/receiver array with 64 hydrophones was deployed in 100 m deep water in an area south of Hudson Canyon off the New Jersey coast. Sets of four, five, or six beams were transmitted at small angles from horizontal and the response from an echo repeater was recorded on 64 channels. Data was collected in 500 Hz bands between 500 and 3500 Hz with the echo repeater at ranges from 0.5 to 5.0 km and at mid‐water column and near‐bottom depths. Applying the broadband DORT algorithm, the isolation of the echo repeater signal in the singular vectors and the robust...